1. Field of the Invention
This invention relates generally to methods for the nonselective etching or polishing of single crystal dielectric materials as opposed to milling or preferential etching. More specifically, it relates to a method for nonselectively etching or polishing single crystals of sapphire (.alpha.-Al.sub.2 O.sub.3) and magnesium spinel (MgO.Al.sub.2 O.sub.3) by immersing wafers of such materials in mixtures of sulphuric and phosphoric acid at relatively low temperatures to obtain acceptable polishing rates.
2. Description of the Prior Art
In the manufacture of semiconductor devices where epitaxial depositions of semiconductor material are utilized to form a substrate on which active devices are formed, great care is taken to provide a surface which is free from pitting, growths and other spurious formations and to provide a surface which is smooth, shiny and absolutely planar. Where the surface upon which epitaxial deposition is to be carried out is a semiconductor, chemical and electrochemical polishing techniques have been developed by the prior art which provide substrate surfaces of the desired characteristics and quality. More recently, however, the semiconductor art has sought to replace the semiconductor substrate with dielectric materials such as sapphire to achieve electrical isolation of active devices which are ultimately formed in a deposited epitaxial semiconductor layer. One material which is undergoing wide experimentation for such purpose is sapphire. Silicon, for example, has been epitaxially deposited on the surface of wafers of single crystal sapphire.
Up until the present, the surfaces achieved by known polishing techniques have been only marginally satisfactory. Further, chemical polishing techniques have been erratic and difficult to reproduce and control. Further still, the semiconductor art has advanced to the point where devices of extremely small size have been fabricated and where devices of even smaller size are being contemplated. The dimensions of the contemplated devices are roughly equivalent to the departure from planarity of portions of the surface of sapphire wafers polished by known techniques. The quality of the subsequently deposited epitaxial layer in which devices are to be formed is extremely sensitive to the surface condition of the substrate. Under such circumstances, using conventional masking, photolithographic and etching techniques, the desired dimensional resolution cannot be obtained and the quality of deposited layers is questionable. One way to obtain the desired dimensional resolution would be to enhance the smoothness and planarity of the substrate upon which epitaxial deposition is to be carried out. Mechanical polishing techniques while providing satisfactory surfaces from the point of view of planarity and surface appearance, provide wafers which have massive mechanical damage to the crystal surface to the point where it is substantially amorphous in nature. Such surfaces are also unsuitable for subsequent expitaxial deposition. Prior chemical polishing art techniques such as heating a sapphire wafer in a reducing atmosphere of hydrogen do not produce the desired surface quality. Immersing a sapphire wafer in pure phosphoric acid at temperatures of 400.degree.C and higher produces unpredictable, insoluble residues and pitting on the surface of a single crystal sapphire wafer. Continued heating of the acid causes polymerization of the acid producing an almost insoluble coating on the surface in addition to other residues. synthetic alumina has been treated with fused anhydrous sodium tetraborate to obtain smooth surfaces at temperatures of 750.degree.-800.degree.C. This technique has not been found to be satisfactory for producing surfaces suitable for semiconductor epitaxy.
Another known technique which was found to be unsatisfactory was the vapor phase etching of sapphire by inorganic fluorides at elevated temperature.
Mixtures of sulphuric acid and phosphoric acid have been shown in issued patents to accomplish the chemical milling of cermets and ceramics such as alumina which is aluminum oxide. One such patent is U.S. Pat. No. 3,042,566 entitled "Chemical Milling", in the name of J. A. Hardy and assigned to the Boeing Airplane Company. The patent makes a point of indicating that it deals with the chemical milling of alumina as opposed to the chemical polishing of alumina. The prior art techniques mentioned hereinabove all dealt with the chemical polishing of a form of aluminum oxide including single crystal sapphire. The patent just referred to operates in a temperature range of 288.degree. to 399.degree.C and utilizes mixtures of sulphuric acid and phosphoric acid to accomplish the chemical milling of alumina. The chemical milling technique is utilized to reduce the weight of certain members of an airplane and no mention is made anywhere in the patent to indicate that single crystal materials were even contemplated. Further, the patent indicates that the minimum temperature must be approximately 600.degree.F (315.degree.C) in order to realize a milling rate of any value. Thus, chemical milling in the patent only occurs at temperatures of 315.degree.C and above whereas in the present disclosure, as will be seen hereinafter, polishing at substantial removal rates take place at 325.degree.C and below for both single crystal sapphire and magnesium spinel. To further indicate the inapplicability of the above-mentioned patent as pertinent prior art, no mention is made in the patent concerning the effect of orientation on surface quality. This is to be expected since the patent is not dealing with single crystal materials. Since the process of the patent is a rather gross process concerned with weight reduction rather than in improving the surface of a wafer on a microscopic level, it is not surprising that a distinction between chemical milling and polishing was made and that the ability to polish using sulphuric and phosphoric mixtures was not even mentioned in the patent.
U.S. Pat. No. 2,650,156 in the name of E. SheltonJones and assigned to Aluminum Company of America suggests the use of a mixture of 1 part phosphoric acid to 2 parts sulphuric acid by volume heated to 125.degree.C for the brightening of aluminum. This patent does not deal with single crystal materials and, further, the brightening action is carried out at a temperature which is far too low for the polishing of single crystal materials.
Thus, while the prior art suggests the use of various mixtures of sulphuric and phosphoric acids for the chemical milling or brightening of aluminum oxide at various temperatures, none of the known prior art teaches the polishing of single crystal sapphire or magnesium spinel in the temperature range taught by the present invention. The surface quality required for epitaxial deposition cannot be provided by any of the known techniques. Also, the known prior art does not teach the specific temperature ranges required or the specific orientations of the crystalline structures upon which polishing can be achieved.